Due to increased interest in using a cellular network, e.g. a Long Term Evolution (LTE) network, for various applications, such as Vehicle-To-Everything (V2X) communication, the Third Generation Partnership Project (3GPP) has defined further functionality that facilitates use of the cellular network for these various applications.
Support for V2X has been defined in 3GPP Release-14 (Rel.14) as extensions to existing Device-To-Device (D2D) specifications. As illustrated in FIG. 1, V2X generally refers to any combination of direct communication between vehicles 112, 114, 115, between a vehicle 112, 114, 115 and a pedestrian 113 with User Equipment (UE) 111, such as a cellular mobile phone and between a vehicle 112, 114, 115 and a network (NW) infrastructure 120, e.g. a base station. V2X communication may take advantage of the NW infrastructure 120, when available, but at least basic V2X connectivity should be possible even in case of lack of coverage by the NW infrastructure 120. Therefore, V2X typically includes a combination of D2D communication and NW infrastructure-based communication. Providing an LTE-based V2X interface may be economically advantageous thanks to potential reuse of existing LTE networks. Moreover, tighter integration between communications with the NW infrastructure (V2I) and Vehicle-To-Pedestrian (V2P) and Vehicle-To-Vehicle (V2V) communications, as compared to using a dedicated V2X technology, may be enabled.
V2X communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g. in terms of latency, reliability, capacity and the like.
In view of the aforementioned D2D communication, also referred to as sidelink operation, sidelink transmission or Proximity-based Services (ProSe, PC), a so called PC5 interface for radio communication have been standardized in 3GPP Rel-12 and onward. In 3GPP Rel-12, two different operation modes have been specified in 3GPP. In a first operation mode (mode-1), a UE in RRC_CONNECTED mode requests D2D resources and a radio base station, or an evolved-NodeB (eNB), grants them via a Physical Downlink Control Channel (PDCCH), while using Downlink Control Information 5 (DCI5) or via dedicated signalling. In a second operation mode (mode-2), a UE autonomously selects resources for transmission from a pool of available resources that the eNB provides in broadcast via System Information Block (SIB) signalling for transmissions on carriers other than a Primary Cell (PCell) or via dedicated signalling for transmission on the PCell. The term PCell is known from a known concept of carrier aggregation, which is not explained here for simplicity. Now, continuing with the operation modes, unlike the first operation mode, the second operation mode can be performed also by UEs in RRC_IDLE.
In Rel.14, D2D communication is extended to include also V2X communication, in order to provide LTE-based D2D communication for V2X services. On the one hand, for design of D2D communication at physical layer in Rel-12, it has been assumed that few UEs compete for the same physical resources in the spectrum, that these UEs carry voice packet for Mission Critical Push-To-Talk (MCPTT) traffic, and that these UEs have low-mobility. On the other hand, V2X communication should be able to cope with higher load scenario, i.e. hundreds of cars could potentially contend physical resources, to carry time/event triggered V2X messages, such as Cooperative Awareness Message (CAM), Decentralized Environmental Notification Message (DENM), and high mobility.
Therefore, 3GPP has discussed possible enhancements to sidelink physical layer, i.e. relating to D2D communication. In particular, in Rel-14 two further operation modes have been introduced: 1) a mode-3 which includes SL SPS and dynamic SL grant similar to mode-1 mentioned above, and 2) a mode-4 which corresponds to UE autonomous resource selection similar to aforementioned mode-2 with some enhancements. These enhancements include a so called sensing procedure in which a UE is required to sense, e.g. listen to, a channel, e.g. one or more resources, for at least a certain time-frame before selecting a resource to be used.
Before proceeding with a description of access control relating to D2D communication, aka sidelink, known access control in LTE will be described.
Access Control in LTE
In case of an overload situation like emergency or congestion, a network may wish to reduce overload by denying access to the cell. Moreover, the network may also need to prioritize between specific users and/or services during overload situations. Access Class Barring (ACB) features have been discussed in 3GPP since Rel-8 in various working groups. Since then, multiple access barring mechanisms have been specified in LTE:    1. ACB as per Rel-8: In this mechanism, it is possible to bar a UE. Normal UEs in Access Class (AC) range 0-9 are barred with a probability factor, also referred to as barring factor and a timer, also referred to as barring duration, whereas specific classes can be controlled separately. Beside the normal classes 0-9, additional classes have been specified to control the access to other type of users, e.g. emergency services, public utilities, security services, etc.    2. Service Specific Access Control (SSAC): The SSAC mechanism allows a network to prohibit Multi-Media Telephony (MMTel)—voice and MMTel-video accesses. The network broadcasts barring parameters (parameters similar to ACB) and an barring algorithm that is similar to ACB (barring factor and random timer). An actual decision if access is allowed is done in the IP Multi-Media Subsystem (IMS) layer of a UE.    3. Access control for Circuit-Switched FallBack (CSFB): The CSFB mechanism allows a network to prohibit CSFB users. A barring algorithm used in this case is similar to ACB.    4. Extended Access Barring (EAB): The EAV mechanism allows a network to prohibit low priority UEs. Barring is based on a bitmap in which each access class (AC 0-9) can be either barred or allowed.    5. Access class barring bypass: The ACB mechanism allows omitting access class barring for IMS voice and video users.    6. Application specific Congestion control for Data Communication (ACDC) barring: ACDC allows barring of traffic from/to certain application. In this solution, applications are categorized based on global application identification (ID) (in Android or iOS). The network broadcasts barring parameters (barring factor and timer) for each category.
Now turning to access control in sidelink, unlike access control in LTE, 3GPP has not yet defined any similar access control framework for the sidelink. Since sidelink operations rely on broadcasting signalling to provide UEs with necessary transmitting/receiving resource pools, a kind of sidelink access control mechanism can in practice be exercised by a network on a cell level by enabling or disabling the broadcasting of transmitting/receiving pools. For example, if sidelink broadcasting signalling, e.g. SIB18 for ProSe communications, SIB19 for ProSe discovery, SIB21 for V2X communications, is not provided, PC5 operations are not allowed in a cell. If sidelink broadcasting signalling is provided, but only a receiving pool is provided, PC5 operations are supported in the cell, but the UE needs to enter RRC_CONNECTED to receive a transmitting pool, e.g. mode-1 or mode-2 as discussed above.
In a context of access control for V2X communication that uses a cellular technology, it may be that for example V2V traffic may use either NW-infrastructure-based communication using LTE-Uu or D2D communication. In case of overload in a cell, a possibility is to use any of the existing mechanisms listed above. However, assuming that the overload may mainly be caused by the V2V communication in this case and recalling that most of the existing access barring mechanisms would also disadvantageously affect other applications than V2X, it may be problematic to use any one of the existing mechanisms.